As is known, at present in the communications systems for cellular telephony the mobile terminals (cellular phones) dialog by means of radio frequencies with the Base Station Systems, hereinafter referred to with the abbreviation BSS (Base Station System), which are connected to the Control Centres, hereinafter referred to with the abbreviation MSC (Mobile Services Switching Centre). The latter allow a connection both to other MSCs, therefore to other mobile phones, and to the fixed network.
The BSSs and MSCs are fixed units and can be connected using optic fibre or with conventional electric cables.
Each BSS consists of a Controller, hereinafter referred to with the abbreviation BSC (Base Station Controller), connected to a plurality of transceivers, hereinafter referred to with the abbreviation BTS (Base Transceiver Station), which basically form the terminals of the fixed part of the network which controls communication between mobile phones.
In order to improve the quality of the signal and cover the widest possible areas, the BTSs are placed in strategic positions, in particular in high places, for example on the top of particularly tall buildings.
With the introduction of third generation (UMTS, Universal Mobile Telecommunications System) mobile phones, this is no longer possible, since the structure of the new base stations is much more bulky and heavy and, as a result, much more difficult to handle from a logistics viewpoint. It is easy to imagine how positioning it on a roof, for example, could be extremely problematic.
Therefore, a slight variation of the conventional structure of the BSSs became necessary, disconnecting the BTSs from the antennas and inserting interface units to control the communications traffic between the BTSs and the remote units (RU—the terminals which comprise the antennas for sending/receiving radio frequency signals to/from mobile phones).
The second section of the connection between the BTSs and RUs, from the interface to the RUs, is normally created using optic fibre with significant advantages in terms of the quality of the communication (low attenuation) and the speed of data transmission.
The data exchange between BTSs and RUs must occur in both directions. The data from the BTSs to the RUs and, as a result, to the mobile phones, is the Down-Link (DL), whilst the signals from the mobile phones received by the RUs and sent on to the BTSs are the Up-Link (UL).
Two different wavelengths are normally used for the above-mentioned data exchange, one for the DL and one for the UL.
A typical example of the connection between the base stations and remote units is the so-called “backbone” configuration. An optic fibre cable runs from the interface unit to all of the RUs and ends close to the last RU. Each RU picks up a portion of the signal present in the fibre, selects the DL wavelength, transforms the optical signal into an RF signal and sends it to the mobile phone by means of an antenna. In parallel, if the RU must send data to the BTS, it sends a UL signal at a preset wavelength, different to that of the DL, in the optic fibre.
In this way, there are two data flows supported by the optic fibre, one from the interface unit to the RUs (DL) and one, in the opposite direction to the first, from the RUs to the interface unit (UL).
The UL and DL can be provided using two physically different supports (one fibre for the DL and another fibre for the UL), without significantly altering the structure and operation of the above-mentioned configuration.
The main disadvantage of this type of connection between the BTSs and RUs is the excessive waste of band in order to set up a bi-directional connection. The connection for each RU uses double the band actually occupied by the signals to be transmitted/received. Too much optic fibre is also used, with significant economic effects on the set up of current systems based on state of the art structures.